1. Field of the Invention
The present invention generally relates to a method for producing a permanent magnet effectively applicable to motors and actuators of various types, and more particularly relates to a method for producing an iron-based rare earth alloy magnet including multiple ferromagnetic phases.
2. Description of the Related Art
Recently, it has become more and more necessary to farther enhance the performance of, and further reduce the size and weight of, consumer electronic appliances, office automation appliances and various other types of electric equipment. For these purposes, a permanent magnet for use in each of these appliances is required to maximize its performance to weight ratio when operated as a magnetic circuit. For example, a permanent magnet with a remanence Br of 0.5 T or more is now in high demand. Hard ferrite magnets have been used widely because magnets of this type are relatively inexpensive. However, the hard ferrite magnets cannot show that high remanence Br of 0.5 T or more.
An Smxe2x80x94Co magnet, produced by a powder metallurgical process, is currently known as a typical permanent magnet with that high remanence Br of 0.5 T or more. Examples of other high-remanence magnets include Ndxe2x80x94Fexe2x80x94B type magnets produced by a powder metallurgical or melt quenching process. An Ndxe2x80x94Fexe2x80x94B type magnet of the former type is disclosed in Japanese Laid-Open Publication No. 59-46008, for example, and an Ndxe2x80x94Fexe2x80x94B type magnet of the latter type is disclosed in Japanese Laid-Open Publication No. 60-9852, for instance.
However, the Smxe2x80x94Co magnet is expensive, because Sm and Co are both expensive materials.
As for the Ndxe2x80x94Fexe2x80x94B type magnet on the other hand, the magnet is mainly composed of relatively inexpensive Fe (typically accounting for 60 wt % to 70 wt % of the total quantity), and is much less expensive than the Smxe2x80x94Co magnet. Nevertheless, it is still expensive to produce the Ndxe2x80x94Fexe2x80x94B type magnet. This is partly because huge equipment and a great number of process steps are needed to separate and purify, or to obtain by reduction reaction, Nd, which usually accounts for 10 at % to 15 at % of the total quantity. Also, a powder metallurgical process normally requires a relatively large number of process steps by its nature.
Compared to an Ndxe2x80x94Fexe2x80x94B type sintered magnet formed by a powder metallurgical process, an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet can be produced at a lower process cost by a melt quenching process. However, to obtain a permanent magnet in bulk by a melt quenching process, a bonded magnet should be formed by compounding a magnet powder, made from a rapidly solidified alloy, with a resin binder. Accordingly, the magnet powder normally accounts for at most about 80 volume % of the molded bonded magnet. Also, a rapidly solidified alloy, formed by a melt quenching process, is magnetically isotropic.
For these reasons, an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet produced by a melt quenching process has a remanence Br lower than that of a magnetically anisotropic Ndxe2x80x94Fexe2x80x94B type sintered magnet produced by a powder metallurgical process.
As disclosed in Japanese Laid-Open Publication No. 1-7502, a technique of adding at least one element selected from the group consisting of Zr, Nb, Mo, Hf, Ta and W and at least one more element selected from the group consisting of Ti, V and Cr in combination effectively improves the magnetic properties of an Ndxe2x80x94Fexe2x80x94B type rapidly solidified magnet. When these elements are added, the magnet can have its coercivity HcJ and anticorrosiveness increased. However, the only known effective technique of improving the remanence Br is increasing the density of a bonded magnet.
As for an Nd-Fe-B type magnet, an alternative magnet material was proposed by R. Coehoorn et al., in J. de Phys, C8, 1988, pp.669-670. The Coehoorn material has a composition including a rare earth element at a relatively low mole fraction (i.e., around Nd3.8Fe77.2B19, where the subscripts are indicated in atomic percentages) and an Fe3B primary phase. This permanent magnet material is obtained by heating and crystallizing an amorphous alloy that has been prepared by a melt quenching process. Also, the crystallized material has a metastable structure in which soft magnetic Fe3B and hard magnetic Nd2Fe14B phases coexist and in which crystal grains of very small sizes (i.e., on the order of several nanometers) are dispersed finely and uniformly as a composite of these two crystalline phases. For that reason, a magnet made from such a material is called a xe2x80x9cnanocomposite magnetxe2x80x9d. It was reported that a nanocomposite magnet like this has a remanence Br of as high as 1 T or more. But the coercivity HCJ thereof is relatively low, i.e., in the range from 160 kA/m to 240 kA/m. Accordingly, this permanent magnet is applicable only when the operating point of the magnet is 1 or more.
It has been proposed that various metal elements be added to the material alloy of a nanocomposite magnet to improve the magnetic properties thereof. See, for example, Japanese Laid-Open Publication No. 3-261104, U.S. Pat. No. 4,836,868, Japanese Laid-Open Publication No. 7-122412, PCT International Publication No. WO 00/03403 and W. C. Chan et. al., xe2x80x9cThe Effects of Refractory Metals on the Magnetic Properties of xcex1-Fe/R2Fe14B-type Nanocompositesxe2x80x9d, IEEE Trans. Magn. No.5, INTER-MAG. 99, Kyongiu, Korea, pp.3265-3267, 1999. However, none of these proposed techniques can always obtain a sufficient xe2x80x9ccharacteristic value per costxe2x80x9d.
An object of the present invention is to provide a method for producing an iron-based alloy permanent magnet, exhibiting excellent magnetic properties including a high coercivity HcJ of e.g., 480 kA/m or more and a high remanence Br of e.g., 0.85 T or more, at a low cost.
An iron-based rare earth alloy magnet according to the present invention has a composition represented by the general formula: (Fe1-mTm)100-x-y-zQxRyMz, where T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C; R is at least one rare earth element substantially excluding La and Ce; and M is at least one metal element selected from the group consisting of Ti, Zr and Hf and always includes Ti. In this formula, the mole fractions x, y, z and m meet the inequalities of: 10 at % less than xxe2x89xa620 at %; 6 at %xe2x89xa6y less than 10 at %; 0.1 at %xe2x89xa6zxe2x89xa612 at %; and 0xe2x89xa6mxe2x89xa60.5, respectively. The magnet has two or more ferromagnetic crystalline phases including hard and soft magnetic phases. An average grain size of the hard magnetic phase is equal to or greater than 10 nm and equal to or less than 200 nm, while that of the soft magnetic phase is equal to or greater than 1 mn and equal to or less than 100 nm.
In one embodiment of the present invention, the mole fractions x, y and z preferably meet the inequalities of: 10 at % less than x less than 17 at %; 8 at %xe2x89xa6yxe2x89xa69.3 at %; and 0.5 at %xe2x89xa6zxe2x89xa66 at %, respectively.
In another embodiment of the present invention, R2Fe14B phase, boride phase and xcex1-Fe phase may coexist in the same metal structure.
Specifically, an average crystal grain size of the xcex1-Fe and boride phases is preferably from 1 nm to 50 nm.
More specifically, the boride phase preferably includes an iron-based boride with ferromagnetic properties.
In this particular embodiment, the iron-based boride preferably includes Fe3B and/or Fe23B6.
In still another embodiment, the mole fractions x and z preferably meet the condition z/xxe2x89xa70.1.
In yet another embodiment, the mole fraction y of the rare earth element(s) R may be 9.5 at % or less.
Alternatively, the mole fraction y of the rare earth element(s) R may also be 9.0 at % or less.
In yet another embodiment, the magnet may have been shaped in a thin strip with a thickness of 10 xcexcm to 300 xcexcm.
In yet another embodiment, the magnet may have been pulverized into powder particles.
Then, a mean particle size of the powder particles is preferably from 30 xcexcm to 250 xcexcm.
In yet another embodiment, the magnet may exhibit hard magnetic properties as represented by a coercivity HcJ of 480 kA/m or more and a remanence Br of 0.7 T or more.
In yet another embodiment, the magnet may also exhibit hard magnetic properties as represented by a remanence Br of 0.85 T or more, a maximum energy product (BH)max of 120 kJ/m3 or more and an intrinsic coercivity HcJ of 480 kA/m or more.
A bonded magnet according to the present invention is formed by molding a magnet powder, including the powder particles of the inventive iron-based rare earth alloy magnet, with a resin binder.
A rapidly solidified alloy according to the present invention is a material for an iron-based rare earth alloy magnet. The alloy has a composition represented by the general formula: (Fe1-mTm)100-x-y-zQxRyMz, where T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C; R is at least one rare earth element substantially excluding La and Ce; and M is at least one metal element selected from the group consisting of Ti, Zr and Hf and always includes Ti. In this formula, the mole fractions x, y, z and m meet the inequalities of: 10 at % less than xxe2x89xa620 at %; 6 at %xe2x89xa6y less than 10 at %; 0.1 at %xe2x89xa6zxe2x89xa612 at %; and 0xe2x89xa6mxe2x89xa60.5, respectively.
In one embodiment of the present invention, the rapidly solidified alloy preferably has a structure, in which substantially no xcex1-Fe phase is included but R2Fe14B compound and amorphous phases are included and in which the R2Fe14B phase accounts for a 60 volume percent or more.
Specifically, the mole fractions x, y and z preferably meet the inequalities of: 10 at % less than x less than 17 at %; 8 at %xe2x89xa6yxe2x89xa69.3 at %; and 0.5 at %xe2x89xa6zxe2x89xa66 at %, respectively. The R2Fe14B phase, accounting for 60 volume percent or more of the alloy, preferably has an average grain size of 50 nm or less.
Another rapidly solidified alloy according to the present invention is also a material for an iron-based rare earth alloy magnet. The solidified alloy is prepared by rapidly cooling a melt of a material alloy comprising Fe, Q, R and Ti, where Q is at least one element selected from the group consisting of B and C; and R is a rare earth element. The solidified alloy has a structure in which an amorphous phase is included and in which heat treatment starts to grow a compound crystalline phase with an R2Fe14B crystalline structure before starting to grow an xcex1-Fe crystalline phase.
An inventive method for producing an iron-based rare earth alloy magnet includes the steps of: preparing a melt of a material alloy that includes Fe, Q, R and Ti, where Q is at least one element selected from the group consisting of B and C, and R is a rare earth element; cooling the melt to make a solidified alloy including an amorphous phase; and heating the solidified alloy to start growing a compound crystalline phase with an R2Fe14B crystalline structure and then an xcex1-Fe crystalline phase.
In one embodiment of the present invention, the melt is preferably cooled by a strip casting process.
Another inventive method for producing an iron-based rare earth alloy magnet includes the step of preparing a melt of a material alloy. The material alloy has a composition represented by the general formula: (Fe1-mTm)100-x-y-zQxRyMz, where T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C; R is at least one rare earth element substantially excluding La and Ce; and M is at least one metal element selected from the group consisting of Ti, Zr and Hf and always includes Ti. The mole fractions x, y, z and m meet the inequalities of: 10 at % less than xxe2x89xa620 at %; 6 at %xe2x89xa6y less than 10 at %; 0.1 at %xe2x89xa6zxe2x89xa612 at %; and 0xe2x89xa6mxe2x89xa60.5, respectively. The method further includes the steps of: rapidly cooling the melt to make a rapidly solidified alloy in which an R2Fe14B crystalline phase and an amorphous phase coexist; and crystallizing the rapidly solidified alloy to form a structure in which two or more ferromagnetic crystalline phases, including hard and soft magnetic phases, exist. An average grain size of the hard magnetic phase is equal to or greater than 10 nm and equal to or less than 200 nm, while that of the soft magnetic phase is equal to or greater than 1 nm and equal to or less than 100 nm.
In one embodiment of the present invention, the rapidly solidified alloy made in the cooling step preferably includes an R2Fe14B phase at 60 volume percent or more.
In another embodiment of the present invention, the cooling step preferably includes rapidly cooling the melt within an ambient gas at a pressure of 30 kPa or more to make a rapidly solidified alloy including an R2Fe14B phase with an average grain size of 50 nm or less.
In this particular embodiment, the cooling step may include: bringing the melt into contact with the surface of a rotating chill roller to obtain a supercooled liquid alloy; and dissipating heat from the supercooled alloy into the ambient gas to grow the R2Fe14B phase after the supercooled alloy has left the chill roller.
In still another embodiment, the method may include the step of heating and crystallizing the rapidly solidified alloy to form a structure in which three or more crystalline phases, including at least R2Fe14B compound, xcex1-Fe and boride phases, are included. In this process step, an average crystal grain size of the R2Fe14B phase is set equal to or greater than 20 nm and equal to or less than 150 nm, while that of the xcex1-Fe and boride phases is set equal to or greater than 1 nm and equal to or less than 50 nm.
Specifically, the boride phase preferably includes an iron-based boride with ferromagnetic properties.
More particularly, the iron-based boride preferably includes Fe3B and/or Fe23B6.
In yet another embodiment, the melt may be cooled by a strip casting process.
An inventive method for producing a bonded magnet includes the steps of: preparing a powder of the iron-based rare earth alloy magnet by the second inventive method for producing the iron-based rare earth alloy magnet; and producing a bonded magnet using the powder of the iron-based rare earth alloy magnet.